Lateritic ores of nickel contain nickel intimately dispersed throughout the ore in the oxide form. Since it is difficult and often impossible to beneficiate such ores by the usual mineral dressing techniques, it is usual practice to subject the entire ore mass to pyrometallurgical or hydrometallurgical processes or combinations of the two.
There are a number of closely related processes employing a combination of pyrometallurgical and hydrometallurgical techniques. These processes have in common the general steps of reducing nickel, and cobalt if present, contained in the ore to the metallic state followed by an oxidative ammonia, or ammonium salt leach. The reduction step is carried out in the solid state using a variety of gaseous, liquid and solid reductants including hydrogen, carbon monoxide, liquid hydrocarbons and coal or coke. In all instances, efforts are made to perform a selective reduction with the object of attaining as complete as possible reduction of nickel to the metallic state while limiting the amount of iron so reduced.
In our past work, represented by U.S. Pat. No. 3,929,468, we found that laterite ores of nickel could be selectively reduced at temperatures in the range of 350.degree. to 600.degree. C using carbon monoxide as the sole reductant to yield nickel in a highly active elemental form. We also found that relatively high concentrations of magnesia in the ore, higher than about 6 to 7%, caused a substantial decrease in the amount of nickel which could subsequently be extracted in an oxidizing ammonium hydroxide-ammonium sulfate leach. Presumably, reduced nickel undergoes a secondary reaction with magnesia and silicon that renders it inert in the subsequent leach. This secondary reaction is inhibited by providing a source of sulfur, such as pyrite, during the reduction. In addition to the previously cited patent, we have published partial results of our research in the Bureau of Mines Report of Investigations 8027 entitled "Recovery of Nickel and Cobalt from Low-Grade Domestic Laterites", April, 1975.